1. Field of the Invention
This invention concerns a two-cycle internal combustion engine and, more specifically, a valve arrangement for use in two-cycle internal combustion engines.
2. Discussion of the Prior Art
In two-cycle piston engines of the prior art, a scavenging port connects the crankcase, via a scavenging passage, to a cylinder in which a piston reciprocates while an exhaust passage is connected to an exhaust port located in the top of the combustion chamber. In such a system, an exhaust valve is seated in the exhaust port and opens at the required exhaust timing during each combustion cycle of the internal combustion engine.
In this type of two-cycle internal combustion engine, just after the combustion and expansion process, the new charge which has been preliminarily pressurized in the crankcase is introduced into the bottom of the combustion chamber through the scavenging port which is opened by the movement of the piston. When the piston begins its upward travel, exhaust gas at the top of the combustion chamber is driven out by the new charge at the bottom of the chamber. Then the new charge in the chamber is compressed and undergoes the combustion and expansion process. Each turn of the crankshaft completes one combustion cycle.
This arrangement permits the new charge which is introduced into the bottom of the combustion chamber to remain as a layer separate from the burned combustion products gas at the top of the chamber, so the latter can be expelled as exhaust. By preventing the new charge from escaping with the exhaust, the fuel efficiency of the engine is enhanced and hydrocarbon emissions are reduced.
However, in the prior art, when the exhaust valve opens during each combustion cycle of the engine, the exhaust valve projects into the center of the combustion chamber, not only causing it to be exposed to high temperature combustion gases, but also causing the exhaust to flow around it. Because of this, a large amount of heat is transmitted to the exhaust valve, which exceed the amount of heat that can be dissipated through the valve seat, whereby the temperature of the exhaust valve could rise to an unacceptable level. Since there is a combustion cycle for each revolution of the engine in a two-cycle internal combustion engine, the above described heat load on exhaust valves is substantial.
Therefore, when such prior art two-cycle internal combustion engines are operated at high output levels such as in outboard motors, the heat load on the exhaust valve can become so high as to cause abnormal combustion or damage to the valve itself, and detract from the longevity and reliability of the engine.